Method for Warm Forming of Super High Tensile Strength Steel Sheet

ABSTRACT

Disclosed herein is a method for warm forming of a super high tensile strength steel sheet. The method may improve productivity and simultaneously maintain accurate strength by securing thermal structure stability and dimensional stability during heating and forming the super high tensile strength steel sheet. According to an exemplary embodiment of the present invention, the method for warm forming of a super high tensile strength steel sheet includes: heating a steel sheet having a single-phase structure at Al transformation temperature or less; pressing, in a mold, the heated steel sheet to be formed in a predetermined shape; and extracting and cooling the pressed steel sheet.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No.10-2014-0139781, filed Oct. 16, 2014, the entire contents of which isincorporated herein for all purposes by this reference.

TECHNICAL FIELD

The present invention relates to a method for warm forming of a superhigh tensile strength steel sheet by simultaneously improving strengthand formability.

BACKGROUND

Recently, use of a super high tensile strength steel sheet has beenincreased with the development of vehicle industries. The super hightensile strength steel sheet refers to a steel material that may havegreater tensile strength than a high tensile strength steel sheet havinga tensile strength of about 50 kg/mm² and may have further improvedstrain hardening, quenching, annealing, tempering, normalizing,precipitation hardening, grain refinement, solid solution strengthening,and the like using various strengthening mechanisms. However, the superhigh tensile strength steel sheet may have reduced formability that isin inverse proportion to the improvement of the strength.

Further, in the related art, the super high tensile strength steel sheetmay not be applied to parts having a complicated shape, parts having adeep forming depth, or the like due to reduced formability of the superhigh tensile strength steel sheet, and thus, many attempts to improvethe formability of the super high tensile strength steel sheet byheating have been conducted.

However, because vehicle part generally require accurate strength andprecise dimensional stability, the super high tensile strength steelsheet may not be used as the vehicle parts due to the reduction instrength , a twist of a member, and a dimensional change thereof.

Recently, in various efforts to solve the above problems, the super hightensile strength steel sheet for vehicle parts has been manufactured bya hot stamping method which may improve formability by heating a steelsheet at a temperature of about 900° C. or greater. Accordingly, thesteel sheet may be formed and obtain high strength by rapidly cooling asteel sheet simultaneously with performing press forming using a specialmold in which cooling water is circulated.

The hot stamping method may provide the steel sheet with an improvedstrength of about 150 K or greater by transforming an austenite phase ofa steel sheet into a martensite phase. The hot stamping method may notcause a spring back even after extracting the steel sheet and mayfurther prevent a dimension of parts from changing, since the partswithin the mold are completely cooled in a fixed state.

However, the hot stamping method may have low productivity to a level of2 SPM, because a specific mold in which the cooling water is circulatedshould be used, laser trimming of a draw panel should be performed, andthe steel sheet is completely cooled within the mold.

Further, since a heating temperature of the steel sheet may increase toan A₃ transformation temperature or greater of austenite, when the superhigh tensile strength steel sheet is processed by the precipitationhardening, strengthening effect of the steel sheet due to precipitatesmay be reduced.

In other words, re-crystallization may occur by high-temperature heatapplied to the steel sheet, and thus most of the strengthening effectsmay be reduced and a lattice structure and a lattice constant may bedeformed depending on a change in a crystalline phase, and as a result,a dimension of the steel sheet may also be changed.

As such, the hot stamping method which has been currently rapidlyapplied may not be considered as an optimal method for manufacturing asuper high tensile strength steel sheet for vehicle parts. Therefore,the present invention may provide a method to solve the above-mentionedproblems.

In the related arts, a certain method for preventing formability of asteel sheet from reducing has been reported in “Steel Strip For TheAutomotive Reinforcement Parts And Method Of Manufacturing Thereof (KR10-0530068)” and “Quenched Steel Sheet Having Ultra High Strength, PartsMade Of It And The Method For Manufacturing Thereof(KR 10-0878614)”.

Further, a CERTAIN method of manufacturing a steel strip for vehiclereinforcement parts has been reported in “Steel Strip For The AutomotiveReinforcement Parts And Method Of Manufacturing Thereof(KR 10-0530068)”.The vehicle reinforcement parts manufactured by the method includingperforming homogenization treatment on aluminum killed steel at 1050 to1300° C., hot rolling thereon at a finishing rolling temperaturecondition of 850 to 950 ° C. which is just above an Ar3 transformationpoint, winding it at a temperature range of 650 to 800 ° C., andperforming cold rolling thereon at a cold reduction ratio of 30 to 80%may have improved heat treatment properties.

Further, in the related art, a quenched steel sheet, parts from thesteel sheet and the method of manufacturing thereof have been developedas reported in “Quenched Steel Sheet Having Ultra High Strength, PartsMade Of It And The Method For Manufacturing Thereof(KR 10-0878614)”. Thequenched steel sheet may have improved tensile strength by heattreatment hardening and then improved yield strength after sufferingfrom painting heat treatment.

However, the above mentioned technical problems such as reducedproductivity due to the use of the specific mold and the rapid coolingin the mold, reduced strengthening effect due to the re-crystallization,change in the lattice constant and the lattice structure, and the likestill remain in the related art.

The above information disclosed in this background section is merely forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY

Thus, in a preferred aspect, the present invention provides a method forwarm forming of a super high tensile strength steel sheet. The methodmay increase productivity and accurate strength, by securing thermalstructure stability and dimensional stability when the super hightensile strength steel sheet is heated and formed.

According to an exemplary embodiment of the present invention, a methodfor warm forming of a super high tensile strength steel sheet mayinclude: heating a steel sheet having a single-phase structure at A_(l)transformation temperature or less; pressing, in a mold, the heatedsteel sheet to be formed in a predetermined shape; and extracting andcooling the pressed steel sheet.

The steel sheet may be a steel sheet having a single-phase martensitestructure or a steel sheet having a single-phase ferrite structure.

The steel sheet having the single-phase ferrite structure may haveimproved strength by precipitation hardening.

A temperature of the mold used in the pressing may be controlled by heattransferred from the heated steel sheet without additional heating andcooling.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is an exemplary graph illustrating elongation and strength of asteel sheet having a multi-phase structure strengthened by astrengthening mechanism for each temperature in the related art;

FIG. 2 is an exemplary graph illustrating that a strength of the steelsheet is changed when the steel sheet having the multi-phase structureand an exemplary steel sheet having a single-phase structure accordingto an exemplary embodiment of the present invention are heated at about400° C. to 700° C. and then cooled; and

FIG. 3 is an exemplary graph illustrating a change in temperature, achange in strength, and a change in elongation of an exemplary superhigh tensile strength steel sheet at the time of warm forming accordingto the exemplary embodiment of the present invention.

DETAILED DESCRIPTION

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g. fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles. The terminology usedherein is for the purpose of describing particular exemplary embodimentsonly and is not intended to be limiting of the invention. As usedherein, the singular forms “a”, “an” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items.

Unless specifically stated or obvious from context, as used herein, theterm “about” is understood as within a range of normal tolerance in theart, for example within 2 standard deviations of the mean. “About” canbe understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%,0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear fromthe context, all numerical values provided herein are modified by theterm “about.”

Hereinafter, an exemplary embodiment of the present invention will bedescribed with reference to the accompanying drawings and background artand reference numerals of the foregoing components are similarly appliedunless specifically mentioned.

A method for warm forming of a super high tensile strength steel sheetdescribed below is an exemplary embodiment of the present invention, andis not limited to the exemplary embodiment but may be implemented invarious forms.

As generally known in the art, for improving strength of metal which iselastically and plastically deformed, atoms may be prevented frommoving. As such, various strengthening mechanisms of strain hardening,quenching, annealing, tempering, normalizing, precipitation hardening,grain refinement, solid solution strengthening, and the like whichrestrict the movement of atoms within a lattice have been used.

When the strengthening mechanism is applied to a steel sheet, a phasechange and a structure change of the steel sheet may internally occurand thus the strength of the steel sheet may be sharply increased, butthe formability may be reduced in inverse proportion to the strength andhave inappropriate characteristics to manufacture parts having acomplicated shape. Accordingly, to apply the steel sheet manufactured bythe strengthening mechanism to the parts having a complicated shape, theformability may be improved by a separate heating process. That is,parts having a predetermined shape may be manufactured by heating thesteel sheet at high temperature to increase the elongation of the steelsheet and then performing the press forming thereon.

However, a phase change, a structure change, a change in a latticestructure, and the like may occur in internal structure of the steelsheet due to high heat applied to the steel sheet and thus thestrengthening mechanism may be reduced and thus, the quenching may beperformed simultaneously with the press forming using a mold in whichcooling water flows to secure the strength of the steel sheet.Accordingly, the present invention provides a method for warm forming ofa super high tensile strength steel sheet. Particularly, in the methodof the present invention, the strengthening mechanism and the latticestructure may not react sensitively to temperature and additionalcooling process such as quenching or water cooling may not be used.

The method for warm forming of a super high tensile strength steelaccording to the exemplary embodiment of the present invention mayinclude a heating process, a press process, and a cooling process. Theheating process may be a process of heating the steel sheet to improvethe formability of the steel sheet and setting the heating temperatureof the steel sheet to be A₁ transformation temperature or less.Mechanical physical properties of the steel sheet may be sharply changeddepending on the heating temperature. For example, when the temperatureof the steel sheet increases, vibration energy of the lattice may beincreased and thus an elasto-plastic behavior may be changed to aviscoelastic behavior and thus the elongation may be increased asillustrated in FIG. 1.

FIG. 1 is an exemplary graph illustrating the elongation and thestrength of the steel sheet which has a multi-phase structurestrengthened by the strengthening mechanism at each temperature of roomtemperature, about 300° C., about 400° C., about 500° C., and about 600°C., respectively. As shown in FIG. 1, when the temperature of the steelsheet is room temperature, the steel sheet may have a strength of about1200 MPa. As the temperature increases, the strength may be reduced andthe elongation may be increased in inverse proportion to the strength.

The increase in elongation of the steel sheet may mean improvement offormability, and thus, when the formality is improved, the parts havinga complicated shape may be easily manufactured by the press forming.However, the steel sheet having the multi-phase structure in whichphases such as ferrite, austenite, and martensite are mixed maydeteriorate from a phase change, a structure change, and a change in alattice structure, and the like as temperature increases. As thetemperature of the steel sheet increases, the strength may be reduced,thereby failing to solve technical problems in the related art.Therefore, in the heating process according to the exemplary embodimentof the present invention, the steel sheet having a single-phasestructure, not the multi-phase structure, may be applied. For example,the steel sheet having the single-phase structure may be heated to a A₁transformation temperature or less.

FIG. 2 is an exemplary graph illustrating that the strength of the steelsheet is changed when the steel sheet having the multi-phase structureand the steel sheet having the single-phase structure are heated atabout 400° C. to 700° C. and then cooled. As shown in FIG. 2, when thesteel sheet having the single-phase structure is cooled, the finalstrength of the cooled steel sheet may not be changed substantiallycompared to initial strength before the steel sheet is heated. Incontrast, as the heating temperature increases, a final strength of thesteel sheet having the multi-phase structure may be sharply reducedafter being cooled.

In other words, when the steel sheet having the single-phase structure,not the steel sheet having the multi-phase structure, is strengthened bythe strengthening mechanism and then is heated at A₁ transformationtemperature or less, both the strength and the formability may beimproved. In particular, heat may be applied until the A₁ transformationtemperature or less of the steel sheet to prevent the dimensionalstability from reducing due to the phenomenon of the phase change, thestructure change, the change in the lattice structure, the change in thelattice constant, and the like which may be caused when the high heat ofthe A₁ transformation temperature or greater applied to the steel sheethaving the single-phase structure. Further, reduction of the strengthdue to reduced strengthening mechanism may be prevented.

In particular, the steel sheet having the single-phase structure mayhave a single-phase structure of martensite or ferrite to provideimproved strength required for the super high tensile strength steelsheet. The martensite may have a needle-shaped structure andsubstantially improved strength and thus may be suitably used for thesuper high tensile strength steel sheet. The ferrite may have relativelyless strength than the martensite but may have improved strength byvarious strengthening mechanisms, and thus, may be applied to the methodfor warm forming of the super high tensile strength steel sheetaccording to the exemplary embodiment of the present invention.

Further, a steel sheet of which the strength is improved by theprecipitation hardening may be applied among the steel sheets having theferrite structure, and thus, the improved strength required for thesuper high tensile strength steel sheet may be obtained. Since theheating temperature is also equal to or less than the A_(l)transformation temperature, the precipitation hardening effect mayremain without deterioration. The press process may form the steel sheetheated by the heating process in a predetermined shape. In particular,when the method for warm forming according to the exemplary embodimentof the present invention is applied, since the strength, the phase, thestructure, the lattice structure, the lattice constant, and the like ofthe steel sheet are not changed, the quenching in the related art maynot be performed.

The press process may use various methods for forming a steel sheet.However, according to an exemplary embodiment of the present invention,using a generally used mold, cooling may not be performed at the time ofthe press forming, and thus cooling time may not be required. Further,the temperature of the mold may be controlled by heat transferred fromthe steel sheet heated without additional heating and cooling, and thusthe generally used mold may be used, unlike a specific mold includingcooling water as used in the related art.

The air cooling process may extract and then cool the steel sheet fromthe press process. In the related art, the steel sheet may be extractedfrom inside of the mold after the steel sheet is cooled to prevent theshape of the pressed steel sheet from changing and the steel sheet fromreducing. However, according to an exemplary embodiment of the presentinvention, extracting and cooling the steel sheet may be completedwithout performing additional cooling after the press process.

Further, as described above, since the strength, the phase, thestructure, the lattice structure, the lattice constant, and the like ofthe steel sheet may not be changed, there is no need of using thecooling method such as cooling water and quenching that requiressubstantial period of time . For example, the air cooling may be appliedeffectively. For example, when the heated and pressed steel sheet iscooled, the steel sheet may be cooled by air as being immediatelyextracted without waiting, and as a result, a production efficiency maybe substantially improved.

FIG. 3 is an exemplary graph illustrating the change in temperature, thechange in strength, and the change in elongation of an exemplary superhigh tensile strength steel sheet according to the exemplary embodimentof the present invention during an exemplary warm forming process at atemperature range of 200° C. to 600° C. according to the exemplaryembodiment of the present invention. Particularly, the temperature maybe maintained less than the A₁ transformation temperature of the appliedsteel sheet. As shown in FIG. 3, when the temperature of the steel sheetincreases, the strength may be gradually reduced to a predeterminedlevel and the elongation may be improved in inverse proportion thereto.

However, when the heated steel sheet is conveyed and mounted in themold, the temperature of the steel sheet may be substantially reduced.Since the additional heating apparatus is not mounted in the mold andthus the temperature of the mold is in a less state than that of theheated mold, when the heated steel sheet contacts the mold, thetemperature may be sharply reduced. Further, to the contrary, thereduced strength may be increased again and thus the predetermined levelmay be maintained, and the elongation may be substantially reduced andthen the predetermined level may be maintained.

The elongation may be substantially reduced and then maintained at thepredetermined level at the time of the press process due to thetemperature of the steel sheet that may be sharply reduced and thenmaintained at the predetermined level. As such, the elongation may bereduced and thus the pressed steel sheet may not be easily deformed andthe shape of the steel sheet may be maintained.

Moreover, when the pressed steel sheet is immediately extracted withoutadditional process and then cooled by air, the temperature of the steelsheet may be reduced to the room temperature over time, and to thecontrary, the strength may be further increased and the elongation maybe reduced, and thus the further forming may not be performed.

As described above, according to various exemplary methods for warmforming of a super high tensile strength steel sheet, improved strengthof the steel sheet may be obtained simultaneously with improvedformability when forming the parts having a complicated shape therebyimproving the productivity.

The present invention may provide various advantages as follows.

First, the present invention may improve the productivity since phasechange may not occur and additional cooling process may not be required.

Second, the present invention may perform the heating at the A₁transformation temperature or less to minimize the waste of heating timeand energy.

Third, the present invention may improve strength while manufacturingthe parts having a complicated shape.

Fourth, the present invention may manufacture the super high tensilestrength steel sheet having substantially improved dimensional stabilityand thermal structural stability.

Fifth, the present invention may maintain the processing hardeningeffect applied to the steel sheet.

The present invention is described in detail with reference to theexemplary embodiments but the scope of the present invention is notlimited to the specific exemplary embodiment but needs to be construedby the accompanying claims. Further, it will be to be understood thatthe present invention may be variously modified and changed by thoseskilled in the art without departing from the scope of the presentinvention.

What is claimed is:
 1. A method for warm forming of a super high tensile strength steel sheet, comprising: heating a steel sheet having a single-phase structure at A₁ transformation temperature or less; pressing, in a mold, the heated steel sheet to be formed in a predetermined shape; and extracting and cooling the pressed steel sheet.
 2. The method of claim 1, wherein the steel sheet has a single-phase martensite structure or a single-phase ferrite structure.
 3. The method of claim 2, wherein the steel sheet having the single-phase ferrite structure has improved strength by precipitation hardening.
 4. The method of claim 1, wherein a temperature of the mold used in the pressing is controlled by heat transferred from the heated steel sheet without additional heating and cooling.
 5. A super high tensile strength steel sheet manufactured by a method of claim
 1. 6. A vehicle part comprising a super high tensile strength steel sheet of claim
 5. 